This invention relates to drying of sludge, and particularly metal hydroxide sludge. Such sludge is a by-product of metal platers and the like, and in its initial form often contains up to as much as 90 percent moisture. For purposes of disposal, it is desired to reduce the moisture content and thus the volume of the sludge.
Heretofore, it has often been the practice to deposit the sludge loosely onto a moving cellular belt and then to pass the belt through an oven wherein the sludge has been subjected to direct heat, such as an open flame directed onto the sludge. This, of course, dehydrates the sludge and reduces its volume.
In recent years it has been determined that metal hydroxide sludge can be a hazardous waste in that it contains minute quantities of heavy metals such as nickel, cadmium, lead, zinc etc. The prior drying systems, such as discussed above, heat the sludge to a very high degree, which can be in excess of 1000 degrees F., causing the metals therein to vaporize into toxic gases. This is unacceptable from an environmental and health standpoint.
It is an object of the invention to improve the manner of deposition of the sludge onto a moving cellular belt. It is a further object of the invention to provide for drying of hazardous sludge in a manner so that the formation of toxic gases is essentially eliminated.
In accordance with the various aspects of the invention, the initially moist sludge is formed into a suitable mass, such as sludge cakes which are fed downwardly in a hopper and pressed into the cells of a multi-cellular moving metal belt in a two-stage process. The first stage deposits the sludge roughly into the cells, while the second stage compresses and compacts the deposited sludge tightly into the cells so that the sludge is in intimate contact with the cell walls throughout each cell.
Furthermore, the sludge-filled belt is passed through a dryer whereby the belt is subject to indirect, rather than direct, heating. More particularly, the belt passes through an enclosed muffle and is caused to be in direct heat-exchanging contact with the bottom muffle wall. The latter wall is heated from beneath and outside the muffle chamber, as by heated air jets. The heat from this wall is not only conducted into the bottom of the sludge passing thereacross, but also into the cell walls of the metal belt itself and hence to the sides of the sludge pieces. The result is efficient sludge drying at acceptably low temperatures.